Work on the aircraft’s cockpit voice recorder will begin “within hours” according to authorities after the devices were recovered from the Mediterranean earlier this month.

The ‘black boxes’ had arrived in France for refurbishment work on Monday including removing salt deposits.

In a statement, Egyptian investigators confirmed the flight data recorder had been "successfully repaired... by the French accident investigation agency laboratory".

It is hoped the breakthrough will allow investigators to analyse data from the Airbus A320 as they try to determine what caused the aircraft to crash.

All 66 people onboard were killed when the aircraft plunged into the Mediterranean en route from Paris to Cairo on May 19.

Meanwhile, a Paris prosecutor opened a preliminary manslaughter investigation into the incident yesterday (June 28).

Spokesperson for the prosecutor’s office, Agnes Thibault-Lecuivre said the inquiry had been launched as an accident investigation not a terrorism case.

Investigators appear to believe that the aircraft was not brought down deliberately.